A Dose-escalation, Expansion Study of ARX788, in Advanced Solid Tumors Subjects With HER2 Expression (ACE-Pan Tumor 01)

Inclusion Criteria: * Age \>18 years * Life expectancy \>3 months. * Female or male subjects whose advanced HER2 expressing cancer has failed standard of care treatments, or for whom such therapy is not acceptable to the subject. Subjects with advanced breast, gastric cancer, or other solid tumor who test positive for HER2 by ASCO/CAP criteria (either IHC or FISH) must have received prior treatment with a trastuzumab containing therapy. Subjects who have been previously treated with pertuzumab, TDM-1, lapatinib, or other available and accessible HER2-directed therapies or investigational therapies are eligible. * Disease measurability: * Phase 1a: measurable or non-measurable disease per RECIST v 1.1. * Phase 1b: measurable disease per RECIST v 1.1 (subjects with non-measurable disease are not eligible for Phase 1b). * Histopathologic evidence of cancer based upon pathology report. * Tumor tissue local laboratory HER2 testing results, adequate tumor sample available for confirmation of HER2 status. Subjects with other types of cancer must have previously tested locally for HER2 status by HER2 IHC or ISH assay. * Phase 1a: ISH positive or IHC 3+ advanced cancer (including breast or gastric/esophageal or other solid tumors). * Phase 1b: Cohort 8 advanced breast cancer (IHC 3+ or IHC 2+/ISH); Cohort 9 advanced breast cancer (IHC 2+ / ISH-); Cohort 10 advanced gastric cancer (IHC 3+ or IHC 2+/ISH+) or gastroesophageal junction adenocarcinoma; Cohort 11 other advanced s…